The present invention relates in general to drive systems for electric vehicles, and, more specifically, to the rapid discharging of capacitors when shutting down the electric drive system.
Electric vehicles, such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), use inverter-driven electric machines to provide traction torque and regenerative braking torque. The inverters typically employ a relatively large energy storage capacitor as a main DC link to maintain a desired bus voltage and absorb switching related ripples. The DC link capacitor is usually interfaced with a high-voltage (HV) battery through a pair of mechanical contactors (e.g., relays).
A shutdown of the electric drive system can result from a vehicle key-off, a high-voltage DC interlock fault, or a vehicle crash, for example. During shutdown, the HV battery is quickly isolated from the rest of the electric system by opening the mechanical contractors. However, there will still be HV electric charge on the DC link capacitor. Due to safety requirements, the HV electric charge should be quickly discharged within a specific time.
The simplest conventional methods for discharging the link capacitor dissipate the charge through a resistance placed across the capacitor. The resistor placement can be passive or active. A passive discharge resistor (PDR), which is hard-wired in parallel with the link capacitor, must have a relatively large resistance to avoid excessive power loss during normal operation. Consequently, it may take one to two minutes to dissipate an HV charge down to a safe level.
In some situations (such as a crash), it may be desirable or required to discharge the capacitor in a much shorter time (e.g., 5 seconds). Therefore, an active discharge resistor (ADR) controlled by a transistor switch may be used so that the charge can be dissipated through a smaller resistance value.
In order to ensure automatic discharge of the link capacitor in the event of a failure of an electronic control unit, the ADR switching device is typically connected to the power bus in a manner that would normally turn it on, and a disable circuit is connected between the control unit and the ADR switching device. The disable circuit keeps the ADR switching device turned off as long as a disable command signal from the control unit is received. If the disable command ceases (either intentionally or as a result of failure of the control unit), the ADR switching device turns on to discharge the link capacitor.
When the disable command signal to the ADR is removed, the electric drive can be in any of several possible states. For example, there may or may not be a source of voltage continuing to charge the capacitor. A voltage could still be supplied to the capacitor even though a shutdown has been attempted if 1) the contactor relays have failed to open and the battery remains connected to the link capacitor, or 2) the vehicle is moving and a back electromotive force (BEMF) from a spinning motor is coupled to the link capacitor. Under these conditions, the ADR dissipates not only the original charge held in the link capacitor but also an ongoing current supported by the continued supply of voltage. Consequently, the prior art ADR circuit has required component power capacities and heat ratings for continuous operation in order survive a worst case scenario. Furthermore, a liquid cooled heat sink to dissipate the continuous heat generated in the ADR resistor may also be needed, which further increases the cost of the circuit.